Using the uniform distribution, we have that:
a) The density curve is given at the end of this question.
b) 16.67% of the time does a customer have to wait between 100 and 120 seconds.
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The uniform distribution has two bounds, a and b, and the probability of finding a value between c and d is given by:
[tex]P(c \leq X \leq d) = \frac{d - c}{b - a}[/tex]
The proportion between 100 and 120 seconds is:
[tex]P(100 \leq X \leq 120) = \frac{120 - 100}{120 - 0} = \frac{1}{6} = 0.1667[/tex]
0.1667*100% = 16.67%
16.67% of the time does a customer have to wait between 100 and 120 seconds.
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